The present invention generally relates to the field of telecommunication services and more particularly, is directed to performance and functionality improvements for broadcast and multicast services using multiple inputs of a waveguide grating router.
As known in the prior art, telecommunications services generally fall into two major categories. There are the so-called broadcast services in which all users receive the same information and the so-called switched services in which each user receives information specific to the specific user. Generally, network infrastructures can also be classified in the same way. An example of a broadcast infrastructure is the classical CATV networks and an example of a switched infrastructure is the public switched telephone network (PSTN). It usually is more economical to deliver broadcast services over broadcast network and switched services over switched networks.
Recent work has shown that the optical properties of certain passive devices can be exploited to permit a given infrastructure to emulate both broadcast and switched. See, for example, U.S. Pat. No. 5,742,414 entitled xe2x80x9cMultiplicity of Services Via a Wavelength Division Routerxe2x80x9d which issued on Apr. 21, 1998. This patent teaches that the cyclical properties of a waveguide grating router (WGR) can be used in conjunction with wavelength division multiplexing (WDM) on several scales of granularity to provide flexible partitioning of both types of networks (broadcast and switched) using the same physical infrastructure. In particular, it is disclosed that by using the cyclical or periodic properties of the WGR (sometimes also called xe2x80x9cArrayed Waveguide Gratingxe2x80x9d (AWG), xe2x80x9cPhased Arrayxe2x80x9d (Phasar), or the xe2x80x9cDragone Routerxe2x80x9d), together with an optical source having a wide spectral emission favors broadcast delivery, while xe2x80x9cline sourcesxe2x80x9d with narrow spectra favors switched service delivery. The use of a wide optical spectrum floods the output optical channels so that each output port carries a replica, or spectral slice, of the signal on the input port. The linear properties of this passive device makes it possible to overlay both broadcast and switched services simultaneously on the same infrastructure.
The ability to segregate such services has been termed xe2x80x9cWDM-on-WDMxe2x80x9d in recognition that a coarser scale of WDM (on the order of the period, or xe2x80x9cfree spectral rangexe2x80x9d of the WGR) can be used to segregate a multiplicity of both broadcast and point-to-point services on an intrinsically xe2x80x9cdensexe2x80x9d WDM infrastructure traditionally used for point-to-point switched services.
Recent work has shown the possibilities of such a system to deliver large quantities of digital TV carriers using a particularly robust QPSK (quadrature phase shift keying) transmission format that permits the use of low quality and potentially inexpensive optical sources with wide optical bandwidths. In particular, it has been shown that both the wavelength domain and the RF domain can be used to deliver xe2x80x9cblocksxe2x80x9d of television programming. These demonstrations have delivered multiple 500 MHz blocks of QPSK modulated carriers from a Satellite service using the location of the optical band and the location of the RF block as a multiplexing index.
Presumably, the user would access this large video content by using a conventional satellite set top box. The set top box would have a front end formed of an optical filter to select the appropriate optical bands and an RF converter to select the appropriate RF carrier blocks. The user can, for a modest increase in cost due to the addition of the optical filter and RF conversion, use a conventional set-top box to access an order of magnitude more video than would otherwise be available to the user. This technique is illustrated in FIG. 1 where each rectangle 1 represents a xe2x80x9cblockxe2x80x9d of service that could be provided to, for example, a conventional set-top box. The ability to provide such increases in capacity for low marginal cost is widely believed to be a necessary characteristic for success in the future for telecommunications operators.
FIG. 2 illustrates how rectangles 1 of FIG. 1 are created. The WDM 2 on the left separates the optical spectrum from the broadband source 16 into four optical bands 3, 4, 5 and 6 corresponding to vertical columns 7, 8, 9 and 10 of FIG. 1. Each of the optical bands is then RF modulated with modulators 11, 12, 13 and 14, respectively, with a composite signal representing the data in 4 independent RF blocks, corresponding to a vertical stack of blocks in FIG. 1. The combined broadcast signal at the output of the second WDM 15 in FIG. 2 is distributed to the end users through a WGR in the field. It should be appreciated that the broadcast signal is usually optically amplified, split and connected to multiple WGRs to achieve the maximum cost sharing of the head end equipment. FIG. 3 generally illustrates broadcast signal distribution to end users through WGRs.
A potential complication of this scheme arises from the fact that when multiple carriers are used in such systems, there are impairments created by the nature of the light itself. In particular, the use of incoherent light with a broad optical spectrum and high frequency modulation exacerbates an impairment known as xe2x80x9cspontaneousxe2x80x94spontaneous beat noise,xe2x80x9d or sometimes referred to as xe2x80x9cexcess noise.xe2x80x9d Generally speaking, the signal-to-noise ratio (SNR) limited by spontaneousxe2x80x94spontaneous beat noise of the blocks as shown in FIG. 1 will be proportional to:
m2(Be/Be) 
where m is the modulation index (per subcarrier) the composite (4 blocks) signal, Bo is the optical bandwidth of the sources (the horizontal width of the box), and Be is the electrical bandwidth of the detected signal (a subcarrier inside one of the boxes in FIG. 1).
In a commercial system, the bandwidth of the QPSK subcarriers is about 30 MHz. Be is given by in accordance with normal television standards. System considerations drives one to reduce Bo (to fit more blocks into the given optical bandwidth of the transport system) and to reduce m (to fit more subcarriers into a vertical stack of blocks.) Considerations of optical noise necessitate that all the blocks be modulated on each vertical stack as a whole, rather than further multiplexing them optically. These factors conspire to make the signal quality deteriorate. What is needed is a way to increase the SNR by increasing the optical bandwidth of the signal.
Accordingly, it is an overall object of the present invention to obviate the above-noted shortcomings and disadvantages of telecommunication services known in the prior art.
A further object of the present invention is to provide an improved network for telecommunication services.
Another object of the present invention is to an provide improved network for telecommunication services which can be easily and inexpensively implemented.
A still further object of the present invention is to provide an improved telecommunications network using optical technology.
It is a specific object of the present invention to provide an improved telecommunications network using optical technology having increased bandwidth over such networks know in the prior art.
It is another specific object of the present invention to provide an improved telecommunications network using optical technology having improved signal-to-noise ratio over such networks known in the prior art.
These another objectives of the present invention are achieved by the present invention as described below.